Flower and Carpel Development

Team FLOCAD / Abdelhafid Bendahmane

The last decades have witnessed an explosion in the genetic and molecular analysis of flowering and the development of a typical hermaphrodite flower. Although most angiosperms develop hermaphrodite flowers, 10% of the species display different sexual morphs.

Monoecious species exhibit male and female flowers on the same plant while dioecious species have separate male and female individuals.

Andromonoecious plants exhibit both male and perfect bisexual flowers. How the gender of a flower or plant is determined is an important question in plant-developmental biology.

Understanding this process also has practical applications, as the gender of a flower or plant often limits how the plant is bred and cultivated. In this axis of research we aim to investigate the development of unisexual flowers and fruit setting in Cucurbitaceae as well as in Solanaceae species.

Through this project, we aim to establish new concepts that will be exploited beyond the investigated species.

Several species in the Cucurbitaceae family, including cucumber and melon, show intraspecific polymorphism in their sexual systems. In these species, floral primordia are initially bisexual with sex determination occurring by the selective developmental arrest of either the stamen or the carpel, resulting in unisexual flowers.

In melon, sex determination is genetically governed by the genes andromonoecious (a) and gynoecious (g), and the interplay of these two genes can result in a range of sexual types. Monoecious (A-G-) and andromonoecious (aaG-) individuals bear male flowers on the main stem and axillary branches and respectively female or hermaphrodite flowers at specific positions on the axillary branches.

Gynoecious (AAgg) and hermaphrodite (aagg) individuals only bear female and hermaphrodite flowers, respectively. In cucumber, three major genes account for most sex phenotypes. The Female (F) gene is a partially dominant gene that controls femaleness.

The androecious gene (a) increases maleness and plants of the aaff genotype are androecious bearing only male flowers.

The Monoecious (M) gene, like the A gene in melon, acts as a stamen suppressor in floral buds determined to develop a carpel. In both Cucumis species, sexual morphs can be modified by hormonal and environmental factors, with ethylene playing a major role.

Consistent with ethylene being a feminizing agent, we previously demonstrated that the melon A gene and the cucumber M gene are orthologs and encode for the rate-limiting enzyme in ethylene biosynthesis, the 1-aminocyclopropane-1-carboxylic acid synthase (ACS). In both species, the ACS genes, referred to as CmACS-7 in melon and CsACS2 in cucumber, are expressed in carpel primordia, and loss of enzymatic activity leads to stamen development.

The F gene in cucumber is also likely to encode an ACS enzyme. The isolation of the gynoecy gene in melon has solved another piece in the puzzle, explaining how hermaphrodite and gynoecious lines develop in melon. The transition from male to female flowers in gynoecious lines results from epigenetic changes in the promoter of a zinc finger type transcription factor, CmWIP1. Expression of CmWIP1 leads to carpel abortion, resulting in the development of unisexual male flowers. CmWIP1 expression also represses indirectly CmACS-7 expression.

Together our data suggested a model where the two genes interact to control the development of male, female and hermaphrodite flowers in melon.

Research Axis 2: A translational Research Platform, to engineer new plant prototypes

Over the last two decades, knowledge about plant growth, development and the molecular compositions of plant organs has increased tremendously. The genes that control the function of the biological mechanisms involved are in many cases identified and well characterized. Unfortunately, development of technologies to manipulate such genes in crop genomes did not match that progress.

To bridge the gap between structural genomics and functional genomics in crop plants, we focused our effort on the development of two complementary reverse genetics tools, TILLING and ECOTILLING. TILLING is based on production of EMS-mutagenised plant collections and rapid systematic identification of mutations in target sequences. ECOTILLING is based on the analysis of germplasm collections to identify natural alleles of a given gene.

Four distinct research areas were at the core of our project :

The production and management of large EMS mutant collections

The development of HTP tools for rapid and systematic identification of mutations

The selection target genes of agronomic importance to be TILLED in each crop

The creation of interactive and evolving databases

Today, the TILLING platform has developed more than 14 mutant collections in 8 crop species and 5 core collections in 4 species.

To manage and integrate the data from both the phenotype recordings and TILLING platform, we implemented the database UTILLdb . Two main types of data are accessible, sequences of tilled genes and the corresponding alleles and when available, the morphological phenotypes of the mutants.